The measurement of near infrared (NIR) spectra is over 200 years old and research into NIR analyzers began over 50 years ago. “The foundations for modern NIR analysis began to be laid in the 1950's when the USDA had embarked on a programme of research aimed at developing chemico-physical methods for rapidly assessing the quality of agricultural commodities.” (Osborne et al., 1993, p 3).
NIR Analyzers have been used for over 30 years to measure constituents of grain, fruit, milk, and meat. After initial applications in the agricultural area, instruments with industrial applications were developed. NIR analyzers for pharmaceuticals, refining, chemical manufacturing, and medical diagnostics emerged.
The function of NIR quantitative analyzers is to predict or estimate the concentrations of various constituents in a substance using the NIR spectra of energy that has passed through at least a portion of that substance. All NIR analyzers perform the same basic operations:                1. Generate energy in some portion of the NIR band (roughly 700 to 2500 nm);        2. Either temporally, spatially, or in combination separate the NIR energy into narrow adjacent bands;        3. Apply energy to the substance under test;        4. Collect a portion of the energy that has passed through part or all of the substance;        5. Measure and record the intensity of the collected energy in one or more of the narrow bands using a suitable detector; and        6. Apply the spectral magnitudes at selected wavelengths as inputs to models developed to predict concentrations of different constituents.        
The second, third, and fourth operations are not necessarily performed in the sequence listed. For example, if the energy is spectrally separated in time only or both in time and space, this is typically done before the narrow band energy is applied to the substance. If instead the energy is only separated spatially, this operation is typically performed on the collected portion of the energy that has passed through the substance.
There are a number of different devices or techniques that can be used to accomplish each of the six main operations of an NIR analyzer:                1. NIR Energy Generation:                    Tungsten Halogen Lamp            Light Emitting Diode (LED)                        2. Spectral Separation:                    Scanning or oscillating diffraction grating            Fixed diffraction grating            Rotating narrow band filter            Rotating variable filter            Fourier Transform (FT) Interferometer            Acousto-Optical Tunable Filter (AOTF)                        3. NIR Energy Delivery to Substance                    Collimating Optics—Direct Output            Collimating Optics with Intervening Fiber Optics                        4. NIR Energy Collection From Substance                    Reflected Energy Focusing Optics—Direct Output            Reflected Energy Focusing Optics with Intervening Fiber Optics            Transmitted Energy Focusing Optics—Direct Output            Transmitted Energy Focusing Optics with Intervening Fiber Optics                        5. Measurement of Collected NIR Energy.                    Single Detector            Detector Array (DA) with Serial Output                        
The detector type used depends on the wavelength range of the analyzer. Common detector types are:                Photomultiplier        Silicon Photodiode        InGaAs photodiode        PbS photoresistor        6. Prediction of Constituent Concentration:        Chemometric Models: MLR, PLS, PCA        Neural Net Models        Genetic Algorithms        Combinations of these        
The energy collected can be broadly categorized as belonging to one of two main types: diffuse reflectance spectra and transmitted spectra. For diffuse reflectance analyzers, the delivery optics and the collection optics are placed on the same side of the substance so that the collection optics receives near infrared radiation reflected diffusely off of the substance being measured. For transmittance analyzers, the delivery optics and collection optics are placed on opposite sides of the substance so that the collection optics receives radiation that has been transmitted through the substance being measured.
Commercial NIR analyzers first appeared in the 1970's in the food and agriculture industry (Osborne et al., 1993, pg. 3). The analyzers were designed to measure various constituents of food such as protein, oil, and starch. For the first decade, all commercial instruments were filter based (Osborne et al., 1993, pg. 5). Filter instruments continued to dominate into the middle of the 1980's:                “The two main methods by which most commercial near-infrared instruments generate wavelengths are the discrete filter and the tilting filter principles.” (Williams and Norris, 1987, pg. 113)        
Although filter instruments dorninated for the first 10 to 15 years in commercial instruments there was another wavelength separation technique that found early favor in research analyzers and on which some commercial units were based. That technique was the scanning monochromator (SM) using a motor driven diffraction grating (Williams and Norris, 1987, pg. 126–127; Burns and Ciurczak, 2001, pg. 61–65).
Just as filter based monochromators dominated initial commercial instruments, reflectance spectroscopy was also favored over transmittance. This was mainly due to the fact that the earlier instruments used wavelengths in the 1200 to 2500 nm range (Naes and Isaksson, 1992, pg. 34).
Diffuse reflectance analyzers have several advantages over transmittance analyzers:                1. Many substances being measured are optically dense (opaque). Therefore, for a given resolution spectrometer and given source intensity, use of transmittance spectra instead of reflectance spectra requires use of thinner samples (short optical path lengths), high powered sources, and/or an integrating detector with long integration times; and        2. The mechanical design of reflectance analyzers is sometimes simpler than transmittance spectrometers as both source and detector are placed on the same side of the sample.        
There are, however, disadvantages to reflectance analyzers. Some of the major disadvantages are:                1. Reflectance spectrometers only measure a thin layer of the surface of the substance being measured. This is a disadvantage if the material is not homogeneous. In contrast, transmittance spectrometers measure the entire body of material;        2. Reflectance spectrometers require use of a separate reference to establish the reference signal. An ideal reference material will diffusely reflect all incident radiation in the wavelength range of interest. Typically the reference material is inserted mechanically between the incoming radiation and the sample window when a reference reading is made; and        3. Reflectance analyzers are affected more than transmittance analyzers by scattering or dusty environments (Osborne et al., 1993, pg. 92–93). This problem is most severe when granular material such as grain is being measured. When a thin layer of dust accumulates on the surface of the sample chamber input window, most of the incident radiation will reflect off of the layer of dust and little off of the grain inside the sample chamber. The acquired spectrum is therefore mainly that of the dust. For transmittance, the dust will reduce the signal. It will also change the spectral signature of the source incident radiation illuminating the grain. But the accumulated dust will also change the spectral signature of the reference signal, which is acquired when the sample chamber is empty. As long as the spectral signatures of the incident radiation penetrating the dust are closely matched for reference and sample signals, normalization will eliminate the dust spectra. This will enable an accurate representation of the grain absorption spectrum to be calculated. For reflectance mode spectrometers placement of the reference material inside the sample chamber is difficult or expensive to do. It is normally placed outside of the sample chamber and mechanically moved in front of the sample chamber window when a reference signal is to be acquired. In this situation, the dust spectra will not be removed by normalization.        
Although filter and scanning grating based analyzers dominated commercial and research applications initially, in recent years other techniques such as Fourier Transform-Near Infrared (FT-NIR) technology in industrial applications and Diode Array based analyzers have emerged. Transmittance analyzers have also been developed in the past 15 years for use in medical, pharmaceutical, and agricultural measurement and control.
In spite of the progress that has been made in the development of NIR analyzers and their many different uses, there are a number of deficiencies that prevent wider application of the technology. New markets await the development of an NIR analyzer with the following features:                1. Low cost;        2. Rugged: operation in presence of dust and vibration;        3. Temperature Stability;        4. Fast Analysis: Take rapid readings even when the intensity of collected energy is low;        5. Operate in transmittance mode analyzing relatively thick samples of optically dense material;        6. High Dynamic Range: Able to measure a wide range (1,000,000:1) of input intensities automatically; and        7. No moving parts.        
An analyzer required to meet all of these criteria precludes the use of many of the devices and techniques used in NIR Analyzers listed above:
1. NIR Energy Generation: Tungsten Halogen Lamps are preferred                LED's in general have insufficient light output and spectral range        
2. Spectral Separation: Fixed Diffraction Gratings are preferred                Scanning gratings, rotating filters, AOTF and FT-NIR are too slow        Scanning gratings, rotating filters, and FT-NIR have moving parts, and are vibration sensitive        Filters are not temperature stable and temperature stabilization is expensive FT-NIR and AOTF are expensive.        
3. NIR Energy Delivery to Substance: Collimating Optics is preferred                Fiber Optics reduces the intensity too much.        
4. NIR Energy Collection From Substance: Collection of Transmitted Energy through focusing optics is preferred                Reflected Energy is susceptible to dust and uses moving parts to measure energy reflected off of reference material        Fiber Optics reduces intensity too much        
5. Measurement of Collected NIR Energy. No Suitable Choice                Single Detector is too slow and requires moving parts. Wavelengths have to be scanned which is too slow, requires moving parts, and is subject to vibration.        Detector Array (DA) with Serial Output: is too slow as output of array has to be scanned serially. The elements of typical diode arrays are too small so that outputs for low level intensity signals are too low.        The detector type used: Silicon is preferred                    Photomultipliers are too expensive            InGaAs photodiode is too expensive, insensitive and requires temperature stabilization            PbS photoresistor is too expensive, insensitive and requires temperature stabilization                        
There are no commercially available spectrometers that can be used to construct an NIR analyzer with the requirements specified above. The closest devices available are the diode array spectrometers such as the S2000 from Ocean Optics. But these suffer from a number of drawbacks. The diode arrays have a large number of photodiodes—from 512 to 2048. This means that the power incident on each photodiode is very low. On top of that, the arrays are scanned serially (as they must be with so many detectors). Thus low intensity signals that would come from NIR energy transmitted through relatively thick samples of optically dense material would take a long time to acquire if they could be acquired at all. In addition the dynamic range of the instrument is limited and would not permit acquisition of a reference signal that is 1000× greater or more in intensity than the sample signal. A neutral density filter would have to be mechanically inserted when the sample chamber is empty in order to acquire and measure the reference signal. Finally, most of the small medium priced diode array based spectrometers are designed only for fiber optic inputs, decreasing the signal strength even further. In summary, the diode array based spectrometers available today are too slow, have too little dynamic range, and do not collect enough energy to meet the specified requirements.